Micromachined ultra-miniature piezoresistive pressure sensor and method of fabrication of the same

ABSTRACT

A method of fabrication of one or more ultra-miniature piezoresistive pressure sensors on silicon wafers is provided. The diaphragm of the piezoresistive pressure sensors is formed by fusion bonding. The piezoresistive pressure sensors can be formed by silicon deposition, photolithography and etching processes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. provisional patent applicationSer. No. 61/768,018, filed on Feb. 22, 2013, which is hereinincorporated by reference.

FIELD OF THE INVENTION

This invention generally relates to micromachined piezoresistivepressure sensor and more particularly to a method of fabricatingmicromachined ultra-miniature piezoresistive pressure sensor for lowpressure applications by applying wafer-bonding technology to formsilicon diaphragms over etched cavities and deep silicon etchingapproach to define the outlines of the fabricated sensors.

BACKGROUND OF THE INVENTION

It is well know that monitoring physiological parameters in situ and invivo within a blood vessel of a human subject being diagnosed or treatedduring a medical procedure can provide medical professionals criticalinformation as to the status and condition of the human subject. Oneparticular important and beneficial physiological parameter is the bloodpressure within the coronary vessel. Study has shown that the bloodpressure in situ and in viva within the coronary vessel can be appliedto calculate Fractional Flow Reserve (FFR) which represents thepotential decrease in coronary flow distal to the coronary stenosis.More than a decade long clinic study has shown that FFR can provide aquantitative assessment of the functional severity of a coronary arterystenosis identified during coronary angiography and cardiaccatheterization, hence, helping the physician to make a quantitativedecision on which kind of further treatment is required.

Due to the small diameter of the coronary vessel, which can be as smallas less than 1 mm in diameter, it is crucial to have a very smalldiagnosis device to measure the blood pressure in situ so the bloodpressure is not distorted by the diagnosis device itself. In such case,an ultra small pressure sensor is mounted at the distal end of an ultrasmall delivering device such as a guide wire to form a sensor guide wireassembly. By advancing the distal end of the sensor guide wire assemblyto the desired location within the coronary vessel, the ultra miniaturepressure sensor mounted at the distal end of the guide wire can performthe measurement of the blood pressure of the coronary vessel in situ andin viva. The Outside Diameter (OD) of the sensor guide wire assembly istypically 0.35 mm. Hence an ultra small sensor is required so the sensorcan be mounted at the distal end of the sensor guide wire assembly.

Therefore it is apparent that there is a need to manufacture miniaturepressure sensors which can be mounted at the distal end of deliveringdevices such as guide wires. In order to mount the pressure sensor on aguide wire whose typical Outside Diameter (OD) is about 0.35 mm, thesize of the sensor needs to be smaller than 0.3 mm in width and 0.1 mmin height. To fabricate pressure sensors with this small dimensionpresents a challenge for mass production.

SUMMARY OF THE INVENTION

This application generally discloses a method of fabricationpiezoresistive pressure sensors. More specifically, it discloses animproved MEMS process for fabricating piezoresistive pressure sensorswhich include a thin film diaphragm overlying a cavity with enhancedprocesses to provide improved sensitivity of ultra miniature pressuresensors which can be mounted on the distal end of a guide wire.

One embodiment provides a piezoresistive pressure sensor fabricatingmethod which can precisely define the dimensions of the pressure sensorsby employing photolithography technology and etching trenches partiallyenclosing the sensors to define the dimensions of the sensors and thesensors can be released by applying mechanical force without dicing.

Another embodiment provides a peiozoresistive pressure sensorfabricating method which can precisely define the dimensions of thepressure sensors by employing photolithography technology and etchingtrenches completely enclosing the sensors to define the dimensions ofthe sensors and the sensors can be released using standard MEMS processwithout dicing.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1A illustrates the top view of an exemplary yet typicalpiezoresistive pressure sensor.

FIG. 1B illustrates the cross-sectional side view of the pressure sensorshown in FIG. 1A. The cross-sectional view section is along line 1A-1A′of FIG. 1A.

FIG. 2A and FIG. 2B illustrates the cross-sectional view of the firstsilicon-on-insulator (SOI) wafer working as a carrier wafer where thecavity is formed on the silicon layer.

FIG. 3 illustrates the cross-sectional view of the second SOI waferwhose silicon layer is used to form the diaphragm in according to oneembodiment of the present invention.

FIG. 4 illustrates the structure after the first and the second SOIwafers are bonded together according to one embodiment of the presentinvention.

FIG. 5 illustrates the structure after the supporting silicon wafer andinsulation layer of the second SOI wafer has been removed according toone of the embodiment of the present invention.

FIG. 6A illustrates the top view of the piezoresistive sensor after theformation of the piezoresistors according to one embodiment of thepresent invention.

FIG. 6B illustrates the cross-sectional view of the piezoresistivesensor after the formation of the piezoresistors. The cross-sectionalview is along line 6A-6A′ of FIG. 6A.

FIG. 7 and FIG. 8 illustrate the steps of forming the piezoresistors.

FIG. 9A through FIG. 10 illustrate the steps of forming the trenchaccording to one or more embodiments of the present invention.

FIGS. 11A and 11B illustrate the steps of removing a bulk silicon layerand an insulation layer of the carrier SOI wafer and release thepiezoresistive sensor according to one or more embodiments of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A and FIG. 1B illustrate a method of manufacturing a miniaturepressure sensor and fabricating one or more piezoresistive pressuresensors 10 using Micro-Electro-Mechanical Systems (MEMS) technology.FIG. 1A illustrates the top view of a piezoresistive pressure sensor.FIG. 1B illustrates the cross-sectional view of the piezoresistivepressure sensor in FIG. 1A. The cross-sectional view in FIG. 1B is alongline 1A-1A′ of FIG. 1A.

The pressure sensor 10 is formed by a thin film diaphragm 13 overlying acavity 11. The dotted line in FIG. 1A represents the edge 16 of thecavity 11 and hence defines the region of the cavity 11. Piezoresistors12 are formed on top of the diaphragm 13 within the cavity 11 region byeither diffusion or ion implantation. When pressure is applied on top ofthe diaphragm 13, the diaphragm 13 will deform and bend into the cavity11, resulting in stress on the piezoresistors 12 hence changing theresistances of the piezoresistors 12.

One important parameter to characterize a pressure sensor is thesensitivity of the sensor. For a given pressure change, the larger theresistance change is, the higher the sensitivity is. The major goal ofthe pressure sensor design is to achieve as high sensitivity aspossible.

The sensitivity of the piezoresistive pressure sensor described above isaffected by the thickness of the diaphragm 13. The thinner is thediaphragm 13, the more sensitive is the pressure sensor. However, thethickness of the diaphragm 13 also affects the pressure range the sensorcan measure. The thinner is the diaphragm 13, the lower pressure thesensor can measure before the change of the resistance of thepiezoresistor 12 becomes nonlinear. It is beneficial is to choose adiaphragm 13 with smallest thickness but large enough for the givendimensions of the cavity 11 so the change of the resistance of thepiezoresistor 12 is linear within all the pressure range the device isdesigned to measure.

The sensitivity of the piezoresistive pressure sensor 10 is alsoaffected by the dimension of the cavity 11. The larger the cavity 11 is,the more sensitive the pressure sensor 10 is. When the space is not alimiting factor, it is beneficial to design the dimension of thepiezoresistive sensor, hence the dimension of the cavity 11, to be largeenough to satisfy the sensitivity requirement. When the space is alimiting factor, such as the case for intra cardiac and intra vascularapplications, the dimension of the piezoresistive sensor, hence thedimension of the cavity 11, is designed to be maximum allowable by thelimited space to achieve maximum achievable sensitivity.

For intra cardiac infra vascular, and Percutaneous Coronary Intervention(PCI) applications, it's preferable to mount the pressure sensor on aguide wire with a Outer Diameter (OD) of about 0.35 mm so no extradevice is required to perform the blood pressure measurement in situ andin vivo. In order to mount the above described piezoresistive pressuresensor on a guide wire, the width of the pressure sensor needs to besmaller than 0.3 mm. This small dimension presents a challenge formanufacturing the above described piezoresistive pressure sensor withtraditionally available MEMS processes which includes a wafer dicingprocess to separate the fabricated devices. The first challenge is thatthe dimension is too small for standard semiconductor dicing equipmentto handle. A semiconductor dicing equipment is only designed to handledevices with lateral dimensions not smaller than 1 mm in any direction.Hence special equipment is required, resulting in increased manufacturecost. The second challenge is caused by the dicing error, which is 0.05mm for most of the commercially available dicing saws. In order toguaranty the finished width is smaller than 0.3 mm, the width of thedevice needs to be smaller than 0.2 mm so after the dicing error itswidth will still be smaller than 0.3 mm. Reducing the width from 0.3 mmto 0.2 mm will result in 33% of sensor area reduction, hence 33% ofsensitivity reduction.

One intention of this invention is to provide a fabrication method offabricating piezoresistive pressure sensor whose dimension can beprecisely defined, hence to eliminate above mentioned problem ofreduction of sensitivity for ultra small piezoresistive pressure sensordue to the uncertainty of the dimension of the finished device. Anotherintention of this invention is to provide a fabrication method offabricating piezoresistive pressure sensor with increased sensitivity.Another intention of this invention is to provide a fabrication methodwhich only uses semiconductor compatible processes to fabricatepiezoresistive pressure sensor. Yet another intention of this inventionis to provide a MEMS process flow which can be used to manufacturepiezoresistive pressure sensor in mass production with high yield andlow cost.

The method of fabrication of piezoresistive pressure sensor whosediaphragm 13 is formed by employing wafer bonding and whose dimension isprecisely defined by employing photolithographic and etching process isdescribed in detail along with the FIGs, in which like parts aregenerally denoted with like reference numbers and letters. The FIGs arefor illustration only and are not to scale.

The steps of fabricating of piezoresistive pressure sensor in accordancewith the present invention employ fusion wafer bonding under vacuum andetching process to define the dimension of the finished pressure sensorsare illustrated and described with reference to FIG. 1 through FIG. 11.The fabrication process starts with two wafers with silicon-on-insulator(SOI) structures thereon.

As illustrated in FIG. 2A, as an example, a first SOI wafer 210, havinga bulk silicon layer 213 thereon, can work generally as a carrier waferfor fabricating the pressure sensor 10. The first SOI wafer 210 may alsoinclude an oxide layer 212 deposited over the bulk silicon layer 213 anda silicon layer 114 deposited over the oxide layer 213.

A cavity 111 is built on the silicon layer 114 of the first SOI wafer210 for fabricating a pressure sensor, such as the pressure sensor 10,therein. The silicon layer 114, the oxide layer 212, and the bulksilicon layer 213 are thus formed into a silicon-on-insulator (SOI)structure on the first SOI wafer 210.

During fabrication of the pressure sensor 10, first, the cavity 111 ofthe pressure sensor is formed on a carrier wafer, such as the first SOIwafer 210. The silicon layer 114 is relatively thick, as it is where thecavity 111 is going to be formed. In one embodiment, the thickness ofthe silicon layer 114 on the first SOI wafer 210 is between about 5 μmand about 100 μm. While the thickness of the silicon layer 114 of thisSOI wafer 210 is not critical to the performance of the piezoresistivepressure sensor 10, the thickness of the silicon layer 114 needs to beshorter than the height limit, if there is any, of the finished sensor10. The thickness of the silicon layer 114 also affects the limit of theheight of the cavity 111 since the cavity 111 needs to be formed withinthe silicon layer 114 of the first SOI wafer 210.

For low pressure applications, such as blood pressure measurement, thecavity 111 does not need to be high. A height of between about 2 μm andabout 50 μm in the cavity 111 is typically sufficient. Hence a thicknessof between about 5 μm and about 100 μm in the silicon layer 114 on thefirst SOI wafer 210 can be used.

As shown in FIG. 2B, a pattern of the cavity 111 can be formed in thesilicon layer 114. In general, a mask with openings is used to form anddefine the shape and pattern of the cavity 111 using photolithographytechniques. The shape of the cavity 111 determines the shape of the areawhere one or more piezoresistors 112 is going to be formed. The mostwidely used shape is square while rectangular shapes, circular shapesand other shapes are also used.

After the photolithograph, plasma etching technique can be used to formthe cavity 111. The time duration and the energy of the plasma etchingdefine the height of the cavity 111. The height of the cavity 111 needsto be much shorter than the thickness of the silicon layer 114, forexample, less than half of the thickness of the silicon layer 114. Thedetailed steps of forming the cavity 111 on the first SOI wafer 210 iswell known to those skilled in the art of the MEMS processes. In somecases, it is possible to directly use a silicon-on-insulator wafer whichhas the cavity 111 already built-in.

FIG. 3 shows one example of using a second SOI wafer 220 to provide athin silicon layer for forming the diaphragm 13 of the piezoresistivepressure sensor 10. The second SOI wafer 220 generally includes a bulksilicon layer 213, an oxide layer 222, and a thin layer of a siliconlayer 113, thereby forming a second silicon-on-insulator (SOI)structure. The thickness of the silicon layer 113 can be as thin asbetween about 0.5 μm and about μm, for forming the diaphragm 13 of thepressure sensor 10.

The thickness of the silicon layer 113 of the second SOI wafer 220defines the thickness of the diaphragm 13. For a given cavity dimension,the thinner the diaphragm 13 is, the more sensitive the pressure sensor10 is to a change in pressure. On the other hand, for the same givencavity dimension, the thinner the diaphragm 13 of the pressure sensor 10is, the smaller the maximum pressure it can tolerate. In general, athickness of the silicon layer and the thickness of the diaphragm 13 arethick enough to be able to tolerate the maximum pressure the pressuresensor 10 is going to be exposed to, resulting in maximum sensitivitywithin the designed pressure range for a given cavity dimension. Forminiature small-scale pressure sensor, such as those suitable forintravascular pressure monitoring where sensitive blood pressure changeis measured, the silicon layer 113 can have a thickness of between about0.5 μm and about 5 μm. Since the silicon layer 113 is provided forforming the diaphragm 13, the type of a silicon material for the siliconlayer 113 is chosen to be opposite to the type of a silicon material forone or more piezoresistors, such as the piezoresistors 12 or one or morepiezoresistors 112, as described below.

When a p-type doping is used to form the piezoresistor 12, a n-typedoped silicon material can be chosen to form the silicon layer 113.Likewise, when a n-type doping is used to form the piezoresistor 12, ap-type doped silicon material can be chosen to form the silicon layer113. This is to limit the leakage from the piezoresistor 12 to thesubstrate of the diaphragm 13. Even though it is not required, it isalso preferable to choose a low impedance silicon material for formingthe silicon layer 113.

Next, as shown in FIG. 4, the second SOI wafer 220 can be placed overthe first SOI wafer 210 with the silicon layer 113 facing the siliconlayer 114, where the cavity 111 of the carrier wafer 210 is. The firstSOI 210 and the second SOI wafers 220 are then bonded together byemploying suitable techniques, for example, fusion wafer bonding undervacuum. If the bonding is done under very low pressure, the formedcavities are vacuum-sealed. In addition, other bonding methods such aseutectic bonding can be applied to bond the first silicon-on-insulatorwafer and the second silicon-on-insulator wafer together.

After bonding and sealing, the bulk silicon layer 223 and the oxidelayer 222 of the second SOI wafer 220 are removed by a grinding processand an etching process, respectively. Accordingly, as illustrated inFIG. 5, only the silicon layer 113 is remained over the surface of thefirst SOI wafer 210, atop the surfaces of the cavity 111 and the siliconlayer 114. The detailed processed to bond the first and the second SOIwafers 210, 220 together using fusion bonding under vacuum and removingthe silicon support wafer 223 and oxide layer 222 is well known to thoseskilled in the art of MEMS process. Reference still to FIG. 5, in somecases, the oxide layer 222 doesn't have to be removed. It can beutilized for future processes if it is desired to do so.

Next, one or more piezoresistors 112 can be formed on top of the siliconlayer 113. FIG. 6A illustrates the top view of the piezoresistive sensor10 after the one or more piezoresistors 112 are formed on a wafer 250.FIG. 6B shows the cross-sectional view, cutting along line 6A-6A′ ofFIG. 6A, of the piezoresistive sensor 10 after forming thepiezoresistors 112.

As illustrated in FIG. 7, first, a thin layer of a silicon dioxideinsulation layer 261 is deposited on top of the silicon layer 113. Thesilicon dioxide insulation layer may have a thickness of around about 1μm. Second, a mask with openings is used to form and define the patternand shape of the one or more piezoresistors 112 using photolithographytechniques.

After photolithography, as illustrated in FIG. 8, a portion of thesilicon dioxide insulation layer 261 in the opening areas is removed,thereby exposing a portion of the silicon layer 113. Then, one or morepiezoresistors 112 are formed on the exposed surfaces of the siliconlayer 113, using diffusion or ion implantation processing techniques.Accordingly, the locations of the piezoresistors 112 are defined by theopening areas in the silicon dioxide insulation layer 261 and should bewithin an edge 116 of the cavity 111 to maximize the sensitivity of thepressure sensor 10. The edge 116 of the cavity 111 is represented by thedotted line 116 in FIG. 6A. In the case where an ion implantationprocess is employed to form the piezoresistors 112, the ion implantationprocess is followed by a high temperature annealing process with typicaltemperature of 800 to 1100 degree Celsius to drive the implanted ionsinto the silicon and to repair the damage on the silicon crystalscreated by the implantation process.

Reference still to FIG. 6A, it should be apparent that the number of thepiezoresistors 112 don't have to be four, even though fourpiezoresistors are illustrated in the figure. In fact, the number ofpiezoresistors 112 can be from one to any number.

Reference still to FIG. 6B and FIG. 7, in the case the oxide layer 222of the second SOI wafer 220 is not removed, the oxide layer 222 can beused directly without depositing a new layer of oxide, hence the stepsdeposition of a thin layer of insulation layer of silicon dioxide 261can be skipped.

Next, one or more trenches 281 are formed to define the boundary of thepressure sensor 10. FIG. 9A illustrates the top view of a sensor 280 onthe first SOI wafer 210, after forming the trenches 281 enclosing one ormore piezoresistors 112. FIG. 9B illustrates the cross-sectional view ofthe wafer 280 after the formation of the trench 281. The cross-sectionalview in FIG. 9B is along line 9B-9B′ of FIG. 9A. FIG. 9C illustratesanother cross-sectional view of the wafer 280 after the formation of thetrench 281. The cross-sectional view in FIG. 9C is along line 9A-9A′ ofFIG. 9A.

First, a mask with openings is used to define the trenches 281, usingphotolithography techniques. The width of the trench 281 can be severalmicrometers to several hundred micrometers and is not critical, as longas the trenches 281 are etched completely through the oxide layer 261,the silicon layer 113, and the silicon layer 114. The boundary of thetrench 281 is outside of the edge 116 of the cavity 111, leaving somespace 285 between the cavity 111 and the trench 281, so that the cavity111 is not affected. The silicon material within the space 285 is goingto support the diaphragm area in the silicon layer 113 after the trench281 has been formed. The width of the space 285 is designed so that thesilicon material remained within the space 285 is strong enough totolerate any stress during the handling a device of the pressure sensor10. The width of the space 285 is typically from 5 μm to 50 μm.

Reference still to FIG. 9A, FIG. 9B and FIG. 9C, in one embodiment, thetrench 281 does not completely enclose the inner sensor area so thesensor 280 is still partially connected across its surface through anarea 282, which may include a thin portion of the silicon dioxide layer261, the silicon layer 113 and the silicon layer 114, as shown in FIG.9A and FIG. 9B. In another embodiment, as shown in FIG. 10, the trench281 can completely enclose the piezoresistive sensor area on the sensor280.

The shape of the area 282, which connects the inner portion of thepressure sensor with the outer surrounding portion of the sensor 280, isdesigned to be in a shape that can be easily broken at the edge close tothe center of the sensor so as to define the dimension of the finishedsensor. One way is to design the shape of the area 282 as a trapezoidshape, whose narrower base is facing the center of the sensor 280 whilethe wider base is facing away from the center of the sensor 280. Theratio of the width of the narrow side to the width of the wider side ispreferably smaller or equal to 0.75. In such a design, when force isapplied on the later steps to release the sensor 280 from the first SOIwafer 210, the narrower side is going to break first, hence a welldefined sensor dimension can be achieved.

The width of the narrower side of the area 282 is not critical as longas it is narrow enough so the area 282 can be easily broken to releasethe sensor 280 from the first SOI wafer 210 at later steps. A typicaldesign width of the narrower side of the area 282 is 10 μm to 50 μm. Theshape of the area 282 does not need to be trapezoid, but any shape whosewidth of the side facing the center of the sensor is smaller than thewidth of the side facing away from the sensor. In addition, the numberof the area 282 is not critical and can be any number, for example,there may be two, as shown in FIG. 9A as an example, or four of the area282 on the two or four sides of the wafer 280. Further, the locations ofthe area 282 are not critical, even though the preferable locations ofthe area 282 is in the middle of the sides of the wafer sensor 280, asshown as an example in FIG. 9A.

After the photolithography, a portion of the silicon dioxide insulationlayer 261, the silicon layer 113, and the silicon layer 114 in theopening area are removed, for example, by an etching process, to formthe trenches 281, stopping at the silicon dioxide layer 212 of the firstSOI wafer 210. Once, a pattern of the trenches 281 is formed, thesilicon dioxide insulation layer 261 can be completely removed, forexample, by an etching process. In some cases, the silicon dioxideinsulation layer 261 is not removed and can also be used for futureprocessing steps.

The next steps are to release the sensors 280 from the first SOI wafer210. In one example, as shown in FIG. 11A, where the trench 281completely encloses the cavity 111 without forming the area 282, thesensor side of the first SOI wafer 210, having one or more sensors 280,is attached to a holding material 290 through glue or other material.The holding material 290 holds the one or more sensors 280 fabricated onthe first SOI water 210 in place. Then, the bulk silicon layer 213 ofthe first SOI wafer 210 is removed.

As shown in FIG. 11B next, the dioxide layer 212 of the first SOI wafer210 is removed. After removing the dioxide layer 212, the sensors 280are only connected with the holding material 290 by glue or othermaterial. The glue, which holds the sensors 280 can be removed, forexample, under ultraviolet (UV) light curing, as glue materials becomenon-adhesive after being exposed to UV light for a certain time.

The holding material 290 can be a transparent material such as glass orplastic materials. After the bulk silicon layer 230 and the dioxidelayer 212 are removed, the sensor device can be exposed to UV light andreleased from the holding material 290. Accordingly, pressure sensorsthus fabricated can be easily released from the holding material 290without dicing.

In the example where the trench 281 does not not completely enclose theinner sensor area where the piezoresistors 112 are located, the area 282may remain near the trench 281 after, the bulk silicon layer 213 and theoxide layer 212 of the first SOI wafer 210 are etched away. The onlymaterial holding the sensors in one piece is the area 282, which isnarrow at the inner edge closer to the center of the sensor. Thus, aforce can be applied by a sharp tool such as a knife to break theconnection and the sensors can be released one by one. Since the innerside edge of the area 282 facing the center of the sensor is smallercompared to the outer side edge of the area 282 facing away from thesensor, the inner side facing the center of the sensor is going to breakfirst when a mechanical force is applied, accomplishing a well definedboundary for the sensor.

From the foregoing, it can be seen that there has been provided a methodto manufacture an ultra-miniature pressure sensor whose dimensions areprecisely defined. In addition, there has been provided a method whichuses only silicon compatible process to manufacture a pressure sensor.Compared with conventional method of fabricating a silicon diaphragmpiezoresistive sensor, the present invention has at least the advantagesof: (1) The dimensions of the finished sensors are precisely defined.The sensitivity of the sensor can hence be optimized. (2) The height ofthe finished sensor is precisely defined. (3) No dicing process isrequired. The sensor can be released by all semiconductor compatibleprocesses.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. A method of fabricating a piezoresistor pressuresensor, comprising: providing a first silicon-on-insulator structurecomprising a first silicon layer deposited on a first oxide layer over afirst bulk silicon layer; providing a second silicon-on-insulatorstructure comprising a second silicon layer deposited on a second oxidelayer over a second bulk silicon layer; bonding the first silicon layerand the second silicon layer together with the first silicon layer andthe second silicon layer facing each other; removing the second bulksilicon layer; removing the second oxide layer, thereby leaving thesecond silicon layer of the second silicon-on-insulator structureoverlying the first silicon layer; forming one or more openings ofpredetermined shape and dimension to expose a portion of the secondsilicon layer; forming one or more piezoresistors on the one or moreopenings of the second silicon layer; forming a patterned trench withinthe second silicon layer and the first silicon layer, stopping at thefirst oxide layer; removing the first bulk silicon layer, therebyforming the piezoresistor pressure sensor within the patterned trench.2. The method of claim 1, wherein the first silicon layer has athickness of between about 5 μm and about 100 μm.
 3. The method of claim1, wherein the second silicon layer has a thickness of between about 0.5μm and about 5 μm.
 4. The method of claim 1, wherein the firstsilicon-on-insulator structure is provided with a patterned cavityalready formed within the first silicon layer.
 5. The method of claim 1,further comprising: forming a patterned cavity on the surface of thefirst silicon layer of the silicon-on-insulator structure.
 6. The methodof claim 1, wherein the patterned trench within the second silicon layerand the first silicon layer is positioned to partially enclose apatterned cavity within the first silicon layer, thereby leaving anarea, the area having a shape on the surface of the second siliconlayer, where an inner edge of the area on the surface facing thepatterned cavity is narrower than an outer edge of the area facing awayfrom the patterned cavity.
 7. The method of claim 6, wherein a ratio ofa first width of the inner edge of the area to a second width of theouter edge of the area is less than or equal to 0.75.
 8. The method ofclaim 1, wherein the patterned trench within the second silicon layerand the first silicon layer is positioned to completely enclose apatterned cavity within the first silicon layer.
 9. The method of claim1, wherein the one or more piezoresistors are formed within the openingsof the seconds silicon layer by diffusion.
 10. The method of claim 1,wherein the one or more piezoresistors are formed within the openings ofthe seconds silicon layer by ion implantation.
 11. The method of claim1, further comprising: forming a silicon oxide insulation layer on thesecond silicon layer after the second oxide layer is removed.
 12. Themethod of claim 1, wherein the silicon oxide insulation layer is removedprior to forming the one or more piezoresistors on the one or moreopenings of the second silicon layer.
 13. The method of claim 1, whereinthe silicon oxide insulation layer is not removed prior to forming theone or more piezoresistors on the one or more openings of the secondsilicon layer.
 14. The method of claim 1, wherein the first siliconlayer of said wafer comprises a p-type silicon material and the secondsilicon layer comprises a n-type silicon material.
 15. The method ofclaim 1, wherein the first silicon layer of said wafer comprises an-type silicon material and the second silicon layer comprises a p-typesilicon material.
 16. The method of claim 1, further comprising;removing the second oxide layer after removing the first bulk siliconlayer.
 17. The method of claim 1, wherein the first silicon layer has athickness of between about 5 μm and about 100 μm.
 18. The method ofclaim 1, wherein the second silicon layer has a thickness of betweenabout 0.5 μm and about 5 μm.
 19. A method of fabricating a piezoresistorpressure sensor, comprising: providing a first silicon-on-insulatorstructure comprising a first silicon layer deposited on a first oxidelayer over a first hulk silicon layer; forming a patterned cavity on thesurface of the first silicon layer of the silicon-on-insulatorstructure; providing a second silicon-on-insulator structure comprisinga second silicon layer deposited on a second oxide layer over a secondbulk silicon layer; bonding the first silicon layer and the secondsilicon layer together with the first silicon layer and the secondsilicon layer facing each other; removing the second bulk silicon layer;removing the second oxide layer, thereby leaving the second siliconlayer of the second silicon-on-insulator structure overlying the firstsilicon layer; forming one or more openings of predetermined shape anddimension to expose a portion of the second silicon layer; forming oneor more piezoresistors on the one or more openings of the second siliconlayer; forming a patterned trench within the second silicon layer andthe first silicon layer, stopping at the first oxide layer; removing thefirst bulk silicon layer, thereby forming the piezoresistor pressuresensor within the patterned trench.
 20. A method of fabricating apiezoresistor pressure sensor, comprising: providing a firstsilicon-on-insulator structure comprising a patterned cavity formed on asurface of a first silicon layer deposited on a first oxide layer over afirst bulk silicon layer; providing a second silicon-on-insulatorstructure comprising a second silicon layer deposited on a second oxidelayer over a second bulk silicon layer; bonding the first silicon layerand the second silicon layer together with the first silicon layer andthe second silicon layer facing each other; removing the second bulksilicon layer; removing the second oxide layer, thereby leaving thesecond silicon layer of the second silicon-on-insulator structureoverlying the first silicon layer; forming a silicon oxide insulationlayer on the second silicon layer after the second oxide layer isremoved; forming one or more openings of predetermined shape anddimension to expose a portion of the second silicon layer; forming oneor more piezoresistors on the one or more openings of the second siliconlayer; forming a patterned trench within the second silicon layer andthe first silicon layer, stopping at the first oxide layer; removing thefirst bulk silicon layer, thereby forming the piezoresistor pressuresensor within the patterned trench.